A cDNA probe has been prepared to each of the known subgroups of feline leukemia virus and to the sarcoma specific sequences of one of the strains of feline sarcoma virus, the Gardner strain. The cDNA probes homologous to the different feline leukemia viruses indicate that certain lymphomas induced in cats by FeLV (feline leukemia virus) contain RNA and DNA sequences not found in normal feline tissue. The sarcoma specific probe prepared from the Gardner sarcoma virus indicates that its oncogenic sequences are distinct from the oncogenic sequences of another strain of feline sarcoma virus, the Snyder-Theilin strain. The heterogeneity of sarcoma specific sequences in an outbred population is similar to previous findings in a murine inbred population. BIBLIOGRAPHIC REFERENCE: Leven, R., Ruscetti, S.K., Parks, W.P. and Scolnick, E.M.: Expression of feline type-C virus in normal and tumor tissues of the domestic cat. Internatl. J. Cancer 18: 661-671, 1976.